1. Field of the Invention
This invention relates to a device for detecting light in the near infrared spectrum utilizing lattice matched InGaAs.
2. Description of Related Art
Photodetectors made of InGaAs lattice matched to InP have been used in many applications from spectroscopy to imaging (M. J. Cohen and G. H. Olsen xe2x80x9cNear-IR Imaging Cameras Operate at Room Temperature,xe2x80x9d Laser Focus World 27,21 (1991)) to communications (H. Haupt, xe2x80x9cInP-based components for telecom systems in Europexe2x80x9d, Proc. of the Ninth International Conference on Indium Phosphide and Related materials, Cape Cod, Mass., 3 (1997)). This material detects light from 0.9-1.7 xcexcm. It has been fabricated into single element detectors, 1-D linear arrays up to 512 elements long, and 2-D arrays as large as 320xc3x97240 elements. (G. H. Olsen, xe2x80x9cInGaAs fills the near-IR detector-array vacuumxe2x80x9d, Laser Focus World,27,21(1991), M. H. Ettenberg, M. Lange, A. R. Sugg, M. J. Cohen, and G. H. Olsen, xe2x80x9cA 2 xcexcm cutoff 320xc3x97240 InGaAs NIR cameraxe2x80x9d, Proc. Of 11th annual IEEE Lasers and Electro-Optics Society, Orlando, Fla., 1092-8081, 71 (1998). The 1-D and 2-D arrays are generally hybridized to Read Out Integrated Circuits (ROIC). These ROIC integrate signal over time. The ROIC does not differentiate between photocurrent and darkcurrent from the detector.
The ROIC applies a limited amount of reverse bias to the detector. Some ROIC""s operate near zero bias (xc2x13 mV) others run as high 5V. The ROIC have storage capacitors at each pixel location and they have limited space for storing charge. Dark current is produced by the detector when it is placed in reverse bias. It is current produced by the device without the device being illuminated. The smaller the amount of dark current produced by the detector allows for larger amounts of photocurrent to be collected before the capacitor fills in the ROIC. This leads to a larger signal-to-noise ratio. It is the goal of the present invention to minimize the amount of dark current produced by the diode at a given applied bias.
Most InGaAs lattice matched to InP is used in the communications industry. InGaAs is capable of detecting 1.55 xcexcm light, which is commonly used in fiber optic communication. The standard photodetector used in these applications is the p-i-n InGaAs structure. Detectors used in fiber optic communications require high speeds. To have fast devices the RC time constant needs to be minimized. If one lowers the doping density of the absorption region in the p-i-n structure, one lowers the capacitance of the device. InGaAs p-i-n diodes are fabricated using epitaxial methods and the lowest doping possible is defined as the background doping or residual doping of the epitaxial method. Much work has been conducted to reduce the background doping and impurities from this layer to make it as pure as possible. Traditionally the InGaAs absorption region is left undoped, with a resulting background doping on the order of 1013xe2x88x921015/cm3.(R. D. Dupis, J. C. Campbell, and J. R. Velebir, xe2x80x9cInGaAs/InP Photodiodes Grown by Metalorganic Chemical Vapor Depositionxe2x80x9d, J. of Cryst. Growth, 77 595-605 (1986).)
The following U.S. patents may be generally relevant to the state of the art and the present inventions.
U.S. Patents entitled xe2x80x9cPIN Photodiode Having a Low Leakage Currentxe2x80x9d (U.S. Pat. No. 4,904,608) and xe2x80x9cPIN Photodiode Having a Low Leakage Currentxe2x80x9d (U.S. Pat. No. 4,999,696) both describe methods to fabricate a low leakage current photodiode in a MESA structure, which is very different from the planar devices described in this disclosure. The patent disclosure stresses that the doping of the layers should be optimum for lowering the capacitance which is equivalent to minimizing the doping in the intrinsic region.
U.S. Patent entitled xe2x80x9cSemiconductor Structure for Photodetectorxe2x80x9d (U.S. Pat. No. 5,214,276) describes a photodetector made of InGaAs but focuses on preventing stray light from entering the diode.
U.S. Patent entitled xe2x80x9cMulti-Layered Semi-Conductor Photodetectorxe2x80x9d (U.S. Pat. No. 4,682,196) describes a structure for making high speed, low dark current devices. This is a very different structure from the structure discussed in this disclosure and it stresses that the intrinsic region should be doped as low as possible.
U.S. Patent entitled xe2x80x9cLow Leakage Current Photodetector Arraysxe2x80x9d (U.S. Pat. No. 5,387,796) describes a method to lower the dark current in non-lattice matched material. In non-lattice matched material the main issue is stress relief due to lattice mismatch between the epitaxial layers and the substrate it is grown on. According to the prior art, p-i-n structures are generally specified to have no intentional doping. They are grown by epitaxy from either the liquid phase or the vapor phase by many methods. In the prior art the doping is described as similar to this disclosure xcx9c1xc3x971015/cm3, but this was achieved utilizing residual doping. The doping of the layers was not intentional. This residual doping was the lowest achievable doping at the time of the prior art. (M. J. Robertson, S. Ritchie, S. K. Sargood, A. W. Nelson, L. Davis, R. H. Walling, C. P. Skrimshire, R. R. Sutherland, xe2x80x9cHighly Reliable Planar GalnAs/InP Photodiodes With High Yield Made By Atmospheric Pressure MOVPExe2x80x9d, Electron. Lett., 24, 5 (1988)). In all of the prior art the material is described as xe2x80x9cundopedxe2x80x9d.
In the prior art low doping allows the intrinsic region to be thick enough to collect all of the light, yet be fully depleted at operating biases so as to minimize the capacitance. The result being high speed (GHz) operation. In the communications industry the dark current of the detector is not critical, speed is the critical factor. In applications where an ROIC is used, the devices speed is no longer critical, the amount of dark current becomes the critical factor. Devices that are hybridized integrate signal over time and thus the device does not run at high speeds. Without the need for high speed, the removal of the dopants and impurities from the absorption layer may not be the best solution.
Briefly described, the invention comprises an In0.53Ga0.47As p-i-n diode. The shunt resistance R0 can be increased by intentionally doping the InGaAs absorption region. According to the present invention, n-type dopants are intentionally introduced at concentration levels between 1xc3x971014 to 5xc3x971017/cm3 in the absorption region of a p-i-n diode. This doping level decreases the dark current at the operating voltages of the ROIC. Lowering of the dark current allows for greater gain from the devices or better signal-to-noise ratio from the device.
The main focus of the invention is to reduce dark current. Higher doping would be unacceptable in traditional high speed InGaAs p-i-n diodes. The increased doping lowers the breakdown voltage and increases the device capacitance lowering its bandwidth. For the relatively slow, low bias conditions used in video-rate imagers and spectroscopy applications these are not relevant. It has been discovered that it is better to intentionally dope the absorption layer to a given concentration for lower dark current in these array type devices.